1. Field of the Invention
This invention relates to monolithic refractory materials used for lining receptacles such as a ladle or tundish or a trough and the like for treating molten metals.
2. Description of the Prior Art
For lining receptacles, troughs and the like for treating molten metals, monolithic refractory materials have widely superseded conventional brick shaped refractory materials which must be in a variety of shapes and require much labor and time for lining. Lining operations for monolithic refractory materials are carried out by various techniques such as spraying, casting, stamping or vibration forming. Vibration forming is accepted as the best lining method because it easily provides the final desired shape, and the molded structure has a uniform texture without the occurrence of lamination or the like.
Monolithic refractory materials for vibration forming are required to have "thixotropy," i.e., upon the application of vibrations the refractory materials decrease in viscosity and become flowable, and upon the removal of the vibrations they are solidified and molded. Monolithic refractory materials that can be used for fibration forming are, for example, described in U.S. Pat. No. 3,892,584. The present invention is an improvement over the invention disclosed in this U.S. patent. Raw materials for the refractory materials usable in the present invention are those which do not react with silica sol or alumina sol at room temperature, for example, silica sand, fused silica, aluminum silicates such as mullite, boron carbide, and silicon nitride, silicon carbide and carbon (including graphite) which are disclosed in the specification of the above U.S. patent, hereby incorporated.
A mixture of such a refractory powder (raw material) with silica sol, alumina sol or a mixture of silica sol and alumina sol as a binder has thixotropy, and undergoes little drying shrinkage and cracking. Hence, it is suitable as a refractory material for fibration forming. Such mixtures, however, pose the following problems.
The binding force of silica sol or alumina sol or a mixture thereof appears to be due to the cohesive force of fine particles when it is dry and to sintering of the fine particles when they are at high temperature. The sintering temperature of silica sol is at least about 1,100.degree. C, and that of alumina sol is at least about 1,200.degree. C. The strength of such a system from the time when it becomes dry until the sintering temperature is reached is not always sufficient. Frequently, after application of a monolithic refractory lining, it undergoes damage during the steps of preheating or transferring or the like before it receives molten metal. In particular, a monolithic refractory lining is considerably consumed by mechanical and thermal shock at the time it first receives molten metal.
Furthermore, silica sol or alumina sol have only a narrow range where they are stable to changes in pH. Silica sol is stable at a pH of 2.7 to 3.1 or 9.0 to 9.8, and alumina sol is stable at a pH of 2.5 to 5.5. Outside these pH ranges, they become gelled and lose their characteristics as aqueous liquids, especially their thixotropy. In order to increase the strength from the stage when they are dry (hereafter the dry period) until the sintering temperature is reached, it may be feasible to use water-soluble salts together with the above-mentioned binder. This, however, causes changes in the pH of silica sol or alumina sol, and the characteristics of aqueous sols are lost due, for example, to gellation, which in turn leads to a failure of vibration forming.